From skipping a warm-up to never stretching, here’s what to change—and what’s no big deal

In a perfect fitness world, you'd warm-up, you'd cool-down, you'd cross-train, you'd do intervals, and, oh, yeah, a laundry fairy would come wash your gear so you never had to wear a sweaty sports bra two workouts in a row. But alas, if you're like most women, you live in a fitness world where managing to cram in a few minutes at the gym is about as good as it gets. And when you do manage to make it to the gym—and log your usual 20 minutes on the treadmill—you wonder if you're sacrificing results or risking injury by always doing the same old thing.

We took a look at seven common fitness habits to see which ones are forgivable—and which ones you should definitely change.

Unless you’re about to compete in an intense activity, skipping a warm-up—a preliminary, easy workout—won’t likely hurt you. And in some cases, a too long warm-up can actually decrease workout performance, finds research published in the Journal of Applied Physiology. According to the study, cyclists who warmed up extensively ended up sacrificing their performance. The athletes faired better with a shorter, more leisurely warm-up.

“It depends on what you’re doing,” says Robert G. Marx, MD, an orthopedic surgeon with the Hospital for Special Surgery in New York City. “You want to warm-up if you’re playing a sport that involves sprinting, such as soccer.” In that case, start with a few minutes of low-intensity dynamic (movement-based) exercise, such as 10 yards of skipping, backwards running, or lunges, says Tom Holland, MS, CSCS, an exercise physiologist and author of The 12-Week Triathlete. For walking or weight training, it's OK to skip the warm-up but start out easy.

Muscle aches occurring a day or two after a strength workout is a sign of delayed onset muscle soreness (DOMS), which can also happen if you’ve tried a new exercise move or worked out more intensely. And it's totally normal, as in no need to ride the couch for days of recovery. “DOMS is believed to be caused by microscopic tears within the affected muscle fibers,” says C. David Geier, Jr., MD, an orthopedic surgeon and director of sports medicine at the Medical University of South Carolina.

And there’s no need to skip your workout entirely, says Dr. Geier. “Simply choose lighter cardio workouts that increase blood flow or practice gentle stretching of the sore muscles.” Just avoid aggravating the sore muscles with the same exercises, Dr. Geier says, as it could cause muscles to remain sore for a longer period of time. The caveat: if you're in pain, not just sore, don't power through. Click through to the next slide to see the difference.

The muscle aches and soreness of DOMS is one thing, but a sharp or persistent pain that worsens over time may be a sign of something more serious. If you continue to push through pain it can worsen over time and be harder to heal, says Dr. Marx. “Generally, you don’t want to work out when something just ‘doesn’t feel right,’ which most people can tell.” Listen to your body. The amount of time off that you lose recovering from an injury is far longer than heeding your body’s warning and going easy in the first place, says Dr. Marx. For example, an ACL tear can take three to four months to heal after surgery, says Dr. Marx. See a doctor if the pain lasts longer than reasonably expected (this varies depending on the injury) or worsens over time.

When you barely have time to work out, cooling down for another 10 minutes seems like time better spent elsewhere. And in most cases it is. Failure to cool down won’t negatively impact you, says Dr. Geier. “However, cooling down for a few minutes allows the heart rate and blood pressure to gradually return to normal and may also keep lactic acid from building up in the fatigued muscles.” A cool-down help flush out the metabolic byproducts that cause that uncomfortable burning sensation in your muscles after a hard workout. And while cooling down isn’t crucial, if flexibility is a goal, you may want to take five minutes at the end of your workout to lightly stretch after a gradual cool down period. “Muscles stretch easier when they’re warm,” says Geier. (Try these great four post-exercise stretches.)

Skipping stretching before exercise is not only forgivable but may even be recommended, according to a recent study published in The Journal of Strength and Conditioning Research. The study shows that stretching prior to weight lifting may make you feel weaker and more off-balance during your workout—two things you could do without when you've got dumbbells teetering over your face. Another analysis of data published in the Scandinavian Journal of Medicine Science and Sport finds that stretching before exercise is generally unnecessary. “Studies also show that stretching does not prevent injury,” says Dr. Marx. “If strength is your goal, you’re better off using the time for strengthening and core exercises.”

That said, it's important to maintain flexibility overall—you just don't have to do it before exercise. Stretch in the morning or before bed (it feels great!) or take a weekly yoga class to limber up. Get started with Stretching 101.

Hopping from machine to machine without a real plan has its pros and cons, says Holland. “On the upside you have built-in ‘muscle confusion,’ which means your muscles won’t adapt easily to your routine,” says Holland. “On the downside, unless you’re an advanced exerciser, you need to develop a sound strength base before you can build on it. Jumping around doesn’t allow for that.” Holland says you need a certain amount of consistency before you make changes if you want to develop lean muscle tone. He recommends sticking with one routine for four to six weeks to develop that benchmark of strength and to learn proper lifting techniques.

Staying within your comfort zone means you’re not challenging yourself enough to create results, says Holland. “We tend to do what we like. But if you only do the exercises you like (which are usually the ones you do well) you’re not going to burn as many calories and you’ll just reach a plateau.” Comfortable exercises such as gentle walking or using light resistance may offer low-level heart benefits but little else. “The truth is, exercise isn’t always fun,” says Holland. “If you want results, a sense of accomplishment is fun.” Holland recommends exercising at about a seven intensity on a scale of one to 10 (for cardio as well as resistance training). “You should be uncomfortable, but it’s not unbearable. For cardio this means you can talk, but it’s difficult,” he says.